1. Field of Invention
This invention relates to scanning systems and methods of operation. More particularly, this invention relates to bar code scanning systems including Automatic Gain Control (AGC) and methods of operation for such scanning systems.
2. Description of the Prior Art
Bar code scanners which utilize AGC for enhanced working range performance tend to slow down the decoding speed. The loss of aggressiveness is due to the standard AGC utilizing a slow time constant in order to track the overall signal strength. When events such as noise glitches or specular reflection set the signal strength up to a maximum value, subsequent valid bar code signals could be heavily attenuated while the signal strength output is slowly settling to a final value. During this period, valid bar code signals may be missed while the gain control of the AGC circuit settles to a final value after the specular reflection. The gain control circuit is implemented with peak detectors that have time constants optimized for one or more scan durations and slow changes in signal strength. When this signal is high, the gain of the signal is lowered and vice-versa. A specular reflection or noise glitch will quickly set the gain control signal to a maximum value. The signal will then begin to decay from the large value with respect to an optimum time constant for normal operation. This high value control signal sets the gain of the VCA too small for proper digitization of subsequent valid bar code signals.
Although the loss of decoding efficiency is noticeable in slower speed, single line "aim-and-shoot" scanners, a user may find acceptable the loss of decoding efficiency. However, for omni-directional scanners, the loss of decoding efficiency is much more severe. An "aim-and-shoot" single line scanner has more opportunities for a good decode than an omni-directional scanner. The single scan line is generally aimed directly at the bar code, whereas only a few scan lines of an omni-directional scan pattern will cross the bar code at any given time. Therefore, after a noise glitch or specular reflection, the omni-directional scanner cannot afford to miss valid bar code signals. The opportunity for an omnidirectional scanner to decode a bar code is relatively infrequent, as compared to a single line scanner operating at the same scan speed.
The following prior art patents are representative of scanning techniques which utilize AGC:
U.S. Pat. No. 5,168,148 to James R. Giebel, issued Dec. 1, 1992, and assigned to the assignee of the present invention, discloses an automatic gain control circuit for dynamically adjusting the gain of a receiver that is used to receive light reflected from a bar code. The AGC provides a fast adjustment to receiver gain so that the receiver can provide a decodable signal quickly. The AGC circuit in addition to sensing the final output of the receiver includes an additional feedback path from an amplifier in the receiver that is located before the final amplification stage. The additional feedback has a negligible attack time, and adjusts receiver gain quickly, thereby minimizing the time required to reach steady state.
U.S. Pat. No. 5,288,983 to A. Nakazawa, issued Feb. 22, 1994, discloses a symbol read device which is capable of reading symbols, regardless of their orientations, by scanning the symbols in different directions. The symbol read device includes a light beam emitter, a scanner which scans the light beam over different positions on a symbol surface having a symbol to be read, a detector which detects light reflected from the symbol surface and generates a detection signal indicative thereof, and a variable gain amplifier which amplifies the detection signal. An automatic gain controller which sets a gain of the variable gain amplifier scan line or scan line group in response to the detection signal. A scan position detector detects a scan position and a controller selects the gain set by the automatic gain controller based on an output signal of the scan position detector.
None of the prior art discloses an intelligent AGC system which uses a processor to maintain a constant gain throughout a single scan and maintain the output within the constant voltage window as the input signal strength changes from scan to scan. Nor does the prior art show a scanning system which is immune to noise and specular reflection that often severely degrades the decoding performance of omni-directional scanners that use an AGC circuit.